1. Field of the Invention
The present invention relates, generally, to injection molding systems, and more particularly, but not exclusively, the invention relates to hot-runner components, particularly manifold bushings and nozzles with separate tips or tip inserts secured by a separate retainer piece.
2. Background Information
The state of the art includes various components, including manifold bushings, nozzles and tips for hot runner injection molding systems. Hot-runner nozzles are typically either a valve-gate style or a hot-tip style. In the valve-gate style, a separate valve stem moves inside the nozzle and tip acting as a valve to selectively start and stop the flow of resin through the nozzle. The valve stem may be guided through the nozzle by a bushing in the manifold against which the nozzle seats. In the hot-tip style, a small gate area at the end of the tip freezes off to thereby stop the flow of resin through the nozzle. The present invention applies to both style nozzles.
Hot runner nozzles often have removable tips, either a single-piece tip or two-piece tip assemblies having an inner tip with an outer tip retaining portion. For two-piece assemblies, the inner tip insert is secured in the nozzle housing by the tip retaining portion that typically threadably engages the nozzle housing.
The tip retaining portion of the two-piece tip assemblies and the single-piece tip includes a nozzle seal near the end adjacent the mold. Since the nozzle seal contacts the mold, which is cold relative to the nozzle tip, it is preferable that the nozzle seal material has low thermal conductivity so that heat from the nozzle and nozzle tip is not transmitted into the mold through the nozzle seal.
In a single-piece tip such as that shown in U.S. Pat. No. 5,507,637, the tip is made entirely of the same material, which can be selected to provide the properties needed for a particular application, such as high wear resistance or thermal properties. However, it may be desirable to have different material properties at different portions of the tip, which cannot be achieved with a single-piece tip made of one material.
In two-piece nozzle tips, such as those described in U.S. Pat. Nos. 5,208,052 and 5,299,928, the outer tip retaining portion is a single piece made of one material, and the inner tip insert is made of another material, typically having higher thermal conductivity such as beryllium-copper alloy or other copper alloys, or a wear resistant material such as a carbide alloy. Typically the outer tip retaining portion is made of stainless or tool steel with a medium thermal conductivity, and for many applications the thermal conductivity of this single-material retainer may be sufficiently low to provide sufficient tip performance. But on other applications less heat transfer between the tip retainer portion and the mold is desired.
U.S. Pat. No. 5,421,716 provides a separate gate insert or seal ring that threadably attaches to the nozzle tip and seals against the mold. The gate insert or seal ring could be made of lower thermal conductivity material than the tip to reduce heat transfer between the tip and the mold. U.S. Pat. No. 5,879,727 provides a threaded insulating portion between the tip and a gate insert which attaches to the insulating portion and seals against the mold. The insulated portion reduces heat transfer between the tip and the mold. While such threaded assembly of components allows disassembly for cleaning, such disassembly may be difficult after parts have been used to process plastic, which can encase the components. Thermal cycling may seize parts together. Also, threading of mating components adds cost to the components.
U.S. Pat. No. 6,009,616 teaches a process for manufacturing an injection molding nozzle by brazing a heater element and a seal ring to the nozzle housing, then brazing a tip insert to the nozzle housing using a second brazing material that melts at a lower temperature than that of the material used for the first braze. However, even the low temperature braze at 850 degrees Fahrenheit is higher than the temperature at which some copper alloys used for nozzle tip components anneal, making the process unsuitable for such materials when strength of the material needs to be maintained.
Manifold bushings, such as those described in U.S. Pat. No. 5,374,182, have a melt channel that directs the flow of molten material through a 90 degree turn. For hot runner systems with valve gate style nozzles, the manifold bushing also receives the valve stem and guides it through a portion of the melt channel. It is important to have a fit between the manifold bushing and the manifold that is tight enough to prevent leakage at the interface of the melt channels of the manifold and the manifold bushing, and it is important that the fit between the valve stem and the manifold bushing allow proper movement of the valve stem in the manifold bushing with minimum leakage of molten material along the valve stem. Manifold bushings made of material such as tool steel can provide the desired guidance, seal and wear-resistance for the valve stem, but because they thermally expand the same as the steel manifold, they are typically sized to require a press fit into the manifold. Manifold bushings made of a material with a higher thermal expansion than that of the steel manifold, such as copper alloys, can be installed without a press fit and rely on their thermal expansion during heat-up to tightly seal against the manifold, but such materials typically may allow too much clearance with the valve stem and not wear as well as steel.
There is a need for components for injection molding systems, such as manifold bushings, one-piece nozzle tips, and tip retainers for a two-piece nozzle tip, to have portions made of different materials to optimize desired material properties at the different portions. There is also a need for such portions to be permanently joined together by a reliable cost-effective process. The present invention provides such components and a manufacturing process for them.